1. Field of the Invention
The present invention relates to an image sensor, and more particularly, to a chip-stacked image sensor obtained by embodying an image sensor cell in two chips and combining the chips with each other.
2. Description of the Related Art
FIG. 1 illustrates a 3T-2S image sensor cell obtained by combining two first-type image sensor cells each including three transistors with each other.
Referring to FIG. 1, a 3T-2S image sensor cell 100 is obtained by combining two first-type image sensor cells 110 and 120 each including three transistors with each other. The 3T-2S image sensor cell 100 includes two photodiodes PD0 and PD1, two charge transmission transistors M101 and M102 respectively connected to the photodiodes PD0 and PD1, a common reset transistor M103, and a common conversion transistor M104. The 3T-2S image sensor cell 100 combines two conventional cells with each other and reduces the total number of transistors by two.
Here, since a reset signal Rx01 applied to a gate of the common reset transistor M103 is enabled once when two charge transmission control signals Tx0 and Tx1 respectively applied to gates of the transmission transistors M101 and M102 are enabled, the reset signal Rx01 is represented by using ‘01 (zero and one)’. A selection signal Sx is enabled once when the two charge transmission control signals Tx0 and Tx1 are enabled.
FIG. 2 illustrates a 3T-2S image sensor cell obtained by combining two second-type image sensor cells each including three transistors with each other.
Referring to FIG. 2, a 3T-2S image sensor cell 200 is obtained by combining two second-type image sensor cells 210 and 220 each including three transistors with each other. The 3T-2S image sensor cell 200 includes two photodiodes PD0 and PD1, two charge transmission transistors M201 and M202 respectively connected to the photodiodes PD0 and PD1, a common reset transistor M203, and a common conversion transistor M204.
In FIG. 1, a terminal of the common reset transistor M103 and a terminal of the common conversion transistor are commonly connected to the selection signal Sx. Unlike FIG. 1, in FIG. 2, a terminal of the common reset transistor M203 is connected to a selection signal Sx, and a terminal of the common conversion transistor M204 is connected to a voltage source Vdd.
FIG. 3 illustrates a 4T-2S image sensor cell obtained by combining two image sensor cells each including four transistors with each other.
Referring to FIG. 3, a 4T-2S image sensor cell 300 is obtained by combining two image sensor cells 310 and 320 each including four transistors with each other. The 4T-2S image sensor cell 300 includes two photodiodes PD0 and PD1, charge transmission transistors M301 and M302 respectively connected to the photodiodes PD0 and PD1, a common reset transistor MC2, a common conversion transistor MC3, and a common selection transistor MC4.
Here, since a reset signal Rx12 applied to a gate of the common reset transistor MC2 is enabled once when two charge transmission control signals Tx1 and Tx2 respectively applied to gates of the transmission transistors M301 and M302 are enabled, the reset signal Rx12 is denoted by using ‘12’. Similarly, a selection signal Sx12 applied to a gate of the common selection transistor MC4 is denoted by using ‘12’.
As described above, it is possible to reduce the number of transistors by two by combining two image sensor cells each including three transistors with each other. It is possible to reduce the number of transistors by three by combining two image sensor cells each including four transistors with each other.
In general, as a pixel size of an image sensor decreases, a photodiode area also decreases. Since the photodiode area has a close relation to an amount of available charge, if the photodiode size decreases, the amount of available charge also decreases. The amount of available charge of the photodiode is a characteristic factor for determining a dynamic range of the image sensor. Reduction in the amount of available charge has direct influence on image quality of a sensor.
As shown in FIGS. 1 to 3, although the number of transistors used for the image sensor cell is reduced by using the common reset transistors M103, M203, and MC2, the common conversion transistors M104, M204, and MC3, and the common selection transistor MC4, areas occupied by metal lines through which the transmission signal Tx, the reset signal Rx, and the selection signal Sx are transmitted are large in a layout. The metal lines reduce an area for enabling photodiodes to receive image signals, thereby causing a disadvantage that is reduction of a photodiode size.